1. Field of the Invention
This invention relates to a flyback power converter with multiple outputs, and more particularly relates to a flyback power converter with output voltage being regulated by using a post regulator.
2. Description of Related Art
Power converters with multiple outputs have been widely applied to various electronic products, such as monitor, mini PC, set-top-box (STB), media player, game player, and etc. The demand of such power converters is large and the market keeps growing. However, the rising of oil price and the increasing of environmental consciousness result in more and more regulations about power saving and energy efficiency, which challenges the manufacturers of such power converters. Among these regulations, the mandatory Appliance Efficiency Regulations adopted by California Energy Commission (CEC) on December 2004 and the 80Plus program focusing on the development of desktop computers and servers are most important.
FIG. 1 is a circuit diagram of a typical flyback power converter with multiple outputs 100. As shown, the flyback power converter 100 has a transformer T, which is composed of a primary winding L0, a first secondary winding L1, and a second secondary winding L2. The primary winding L0 is connected to a primary side circuit 110 for accessing an input voltage V0. The first secondary winding L1 is connected to a first output circuit 120 for generating a first output voltage V1. The second secondary winding L2 is connected to a second output circuit 140 for generating a second output voltage V2. The voltage levels of the first output voltage V1 and the second output voltage V2 depend on the number of windings of the first secondary winding L1 and that of the second secondary winding L2.
The primary side circuit 110 has a main switch Q connected between the primary winding L0 and a ground for adjusting the power transmitted from the power input to the transformer T. The first output circuit 120 has a rectifier 122 coupled to the first secondary winding L1. The voltage level of the output voltage V1 of the first output circuit 120 corresponds to the voltage drop across the capacitor of the rectifier 122. The second output circuit 140 also has a rectifier 142 coupled to the second secondary winding L2.
As to the feedback control of the first output circuit 120, the flyback power converter 100 has a feedback circuit 180 and a pulse width modulation (PWM) controller 190. The feedback circuit 180 detects the first output voltage V1 and generates a main feedback signal to the PWM controller 190 accordingly. The PWM controller 190 generates a modulation signal according to the main feedback signal to control on-time of the main switch Q so as to adjust the power accessed by the transformer T and stabilize the level of the first output voltage V1. However, because the second output voltage V2 can not be stabilized by the main feedback signal, the level of the second output voltage V2 would be shifted attending with the variation of input power and output load. For example, the increasing of current outputted from the second output circuit 140 may result in voltage drop of the second output voltage V2; and the increasing of current outputted from the first output circuit 120 may result in the increasing of duty cycle of the main switch Q to raise the level of the second output voltage V2.
In order to stabilize the output voltage other than the main output voltage V1, referring to FIG. 1, the second output circuit 140 of the power converter 100 adopts a regulator circuit 144, such as a linear drop-out (LDO) circuit, serially connected to the rectifier 142 to stabilize the output voltage V2. However, the regulator circuit 144 causes significant energy loss during voltage stabilization. The power converters with such circuit design may encounter poor energy efficiency or even be disqualified for the energy regulations. In addition, as the first output circuit 120 is heavily loaded, the voltage level transmitted to the regulator circuit 144 is enhanced and the potential difference between the input and the output of the regulator circuit 144 is increased, which may result in a greater energy loss during voltage-stabilization.
Accordingly, how to provide multiple outputs with stabilized voltage level and prevent the increasing of energy loss, is an urgent issue to be solved.